Tegmark was born in Sweden, son of Karin Tegmark and Harold S. Shapiro, studied at the Royal Institute of Technology in Stockholm, and later received his Ph.D. from the University of California, Berkeley. After having worked at the University of Pennsylvania, he is now at the Massachusetts Institute of Technology as an Associate Professor. Tegmark belongs to the scientific directorate of the Foundational Questions Institute.

As part of the Sloan Digital Sky Survey team, he has worked on data analysis, extracting the parameters of the Lambda-CDM model from observational large-scale structure and cosmic microwave background data.

He is one of the proponents of the quantum suicide thought-experiment, and has come up with a mathematical argument for the multiverse. The computational expression of a single random number between one and zero (with all its infinite decimals) is longer than the computational expression of the whole set of numbers that exist between 1 and 0, so it may be more informationally economical for reality to consist of infinite parallel universes instead of just one. The computer code for such a computation is only two lines long.

He has also been a strong critic of those who would infer a theory of consciousness from quantum effects, such as Roger Penrose and Stuart Hameroff.

Tegmark has also formulated the "Ultimate ensembletheory of everything", whose only postulate is that "all structures that exist mathematically exist also physically". This simple theory, with no free parameters at all, suggests that in those structures complex enough to contain self-aware substructures (SASs), these SASs will subjectively perceive themselves as existing in a physically "real" world.

He is married to astrophysicist Angelica de Oliveira-Costa and they have two sons, Philip and Alexander.

In 2006, Tegmark was one of fifty scientists interviewed by New Scientist about their predictions for the future. His prediction: "In 50 years, you may be able to buy T-shirts on which are printed equations describing the unified laws of our universes." [1]